I'm not sure how to answer your question as most E46's are still under warranty (not all but most). I have a 2002 330xi which is still under warranty for more than a year or more (too lazy to check). So I don't know where those figures are from of high expense to maintain. So far I have paid for a new windshield wiper blade, that's it. Oh yes, I had an Ipod adapter installed but that doesn't count. Other than gas money, I haven't spent anything.
I will be sharking it however since I am putting in 92 gas I figure I might as well get the software that will take advantage of that.
These cars - the E46's - have had superlative reviews. Even if your repair bills are slightly higher (don't know why) you'll have to wipe the smile off your face after driving it.